Scanning electron microscope and sample observation method using the same

ABSTRACT

According to the present invention, there are newly provided in a scanning electron microscope with an in-lens system a first low-magnification mode that sets the current of the object lens to be zero or in a weak excitation state, and a second low-magnification mode that sets the current of the object lens to be a value that changes in proportion to the square root of the accelerating voltage. The scanning electron microscope has a configuration wherein normal sample image (secondary electron image) observation is performed in the first low-magnification mode, and it switches the first low-magnification mode to the second low-magnification mode when X-ray analysis is performed. As a result, both sample image (secondary electron image) observation and X-ray analysis can be performed in low-magnification mode.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a scanning electron microscope,and particularly to a scanning electron microscope that allows efficientobservation of a magnified sample image (high-magnification image) and awhole sample image (low-magnification image).

[0002] In a scanning electron microscope, the object lens isconventionally used at a very short focal distance to obtain scannedimages of higher resolution, as is typified, for example, by the in-lenssystem in which a scanned image is obtained by placing the samplebetween the magnetic poles of the object lens. When the object lens isused at a short focal distance and sample observation is to be performedunder high magnification, there is employed a lens control method forscanning by a primary electron beam wherein deflecting coils for thescanning by the primary electron beam are arranged in two stages alongthe optical axis, and the deflection point of the primary electron beamis set to be in the proximity of the principal plane of the object lens.The above arrangements are provided in order to prevent distortioncaused by the object lens or an increase in the beam diameter of theprimary electron beam on the periphery of the scanning region. The lenscontrol method described above is called high-magnification mode. On theother hand, when a view search under low magnification is to beperformed at a stage before proceeding to high-magnification observationas described above, or the whole image of the sample is to be observedunder low magnification, the following lens control method is employedto enable scanning of a wide region (low-magnification state) by aprimary electron beam. That is, the exciting current of the the objectlens is set to be zero or in a weak excitation state, and the scanningof the sample by a primary electron beam is performed by using aone-stage deflecting coil or two-stage deflecting coils wherein thedistance between the deflection point and the surface of the sample isset to be longer than that of high-magnification mode. This lens controlmethod is called low-magnification mode. Thus, observation in a widemagnification range from high to low magnification has been madepossible by switching between two magnification modes depending on theobservation magnification.

[0003] Now, in the X-ray ananlysis of the sample by means of a scanningelectron microscope, X-rays occurring from within the scanning range ofthe primary electron beam (view) are detected to identify theconstituent elements of the sample in the view by using an X-rayspectrum and collect information on how the constituent elements aredistributed in the view (X-ray mapping image) and the like. An X-raymapping image, in particular, requires not only local element mapping ofthe sample through high-magnification observation but also generalelement mapping of the sample through low-magnification observation.

[0004] In the case of X-ray observation by using a scanning electronmicroscope, if an X-ray detector can be placed in the proximity of thesample, the extraction angle of X-rays can be increased, and thereforehighly efficient X-ray analysis is performed. However, if a reflectedelectron that occurs from the sample by irradiation with the primaryelectron beam as in the case of X-rays falls on the detection plane ofthe X-ray detector, it may cause an error or a failure. This problemneeds to be avoided.

[0005] If the sample is placed within the magnetic field of the objectlens, as in the case of the in-lens system, X-ray analysis can beperformed in high-magnification mode where the object lens is used in astrongly excitated state. This is because the magnetic field of theobject lens causes the trajectory of the reflected electron to go awayfrom the detection plane of the X-ray detector. However, inlow-magnification mode where the exciting current of the object lens isset to be zero or in a weak excitation state, it is not possible togenerate an object lens magnetic field strong enough to cause thetrajectory of the reflected electron to go away from the detection planeof the X-ray detector. Therefore X-ray analysis is difficult to performin this case.

[0006] Methods for performing X-ray observation in low-magnificationmode include a method in which the X-ray detector is moved so as to keepaway from the trajectory of the reflected electron and a method in whicha magnet is placed on the detection plane of the X-ray detector to causethe trajectory of the reflected electron to go away from the detectionplane of the X-ray detector. However, it is difficult to adopt suchmethods in the in-lens system because of its structure.

SUMMARY OF THE INVENTION

[0007] According to the present invention, low-magnification modecapable of X-ray observation is set in addition to the conventionallow-magnification mode suitable for sample image (secondary electronimage) observation, and X-ray analysis or particularly X-ray mappingimages with a wide view can be obtained by switching between thesemodes.

[0008] According to the present invention, there are provided a firstlow-magnification mode wherein the current of the object lens is set tobe zero or in a weak excitation state, and a second low-magnificationmode wherein the current of the object lens is set to be a value thatchanges in proportion to the square root of the accelerating voltage tobe used. A scanning electron microscope according to the presentinvention is thus provided with a configuration that makes it possibleto switch to the first low-magnification mode when normal sample image(secondary electron image) observation is performed and switch to thesecond low-magnification mode when X-ray analysis is performed.

[0009] Specifically, a scanning electron microscope according toembodiments of the present invention comprises an electron source, afirst focusing lens for focusing a primary electron beam emitted fromthe electron source, an object lens diaphragm for removing anunnecessary region of the primary electron beam focused by the firstfocusing lens, a second focusing lens for focusing the primary electronbeam that has passed through the object lens diaphragm, an object lensfor focusing the primary electron beam focused by the second focusinglens on a sample, a deflecting means for the scanning of the sample bythe primary electron beam, a secondary electron detector for detecting asecondary electron emitted from the sample due to electron beamirradiation, and an X-ray detector for detecting an X-ray emitted fromthe sample. The scanning electron microscope has functions of focusingthe primary electron beam on the sample by using the object lens whenthe magnification of an image to be scanned is higher than a presetvalue (high-magnification mode), and focusing the primary electron beamon the sample by using the second focusing lens when the magnificationof an image to be scanned is lower than a preset value(low-magnification mode. The scanning electron microscope also has aconfiguration that, in low-magnification mode, makes it possible toswitch to a first low-magnification mode in which the exciting currentof the object lens is set to be a constant value independently of theaccelerating voltage of the primary electron beam, and switch to asecond low-magnification mode in which the exciting current of theobject lens is changed as a function of the accelerating voltage of theprimary electron beam.

[0010] The object lens diaphragm limits the focusing angle (aperture) ofthe primary electron beam on the sample. In addition, the control of theprobe current is performed through the control of the focusingconditions of the first focusing lens. The first focusing lens or thesecond focusing lens may be formed by a lens in one stage or lenses in aplurality of stages.

[0011] The sample is placed in the magnetic field of the object lens.For this kind of object lens, there is known a type of object lenscalled an in-lens or a type of object lens called a snorkel lens.

[0012] The first low-magnification mode sets the exciting current of theobject lens to be zero or in a weak excitation state. In other words,the first low-magnification mode sets the exciting current of the objectlens to be the minimum exciting current of the object lens that does notlower the efficiency of secondary electron detection, or the excitingcurrent of the object lens that provides the maximum view for theobservation magnification. The exciting current of the object lens inthe second low-magnification mode is set to be a value in proportion tothe square root of the accelerating voltage of the primary electronbeam.

[0013] The first low-magnification mode and the second low-magnificationmode can be configured in such a way that switching between the firstlow-magnification mode and the second low-magnification mode isperformed automatically according to the set value of magnification inlow-magnification observation. In the first low-magnification mode, theexciting current of the object lens is lower than that of the secondlow-magnification mode, and the brightness region is wider than that ofthe second low-magnification mode (Observation under lower magnificationis possible). Therefore, the range of observation magnifications iswidened by setting a threshold value of observation magnification in thescanning electron microscope in advance so that it switches to the firstlow-magnification mode if a desired observation magnification is lowerthan the threshold value, and it switches to the secondlow-magnification mode if a desired observation magnification is higherthan the threshold value.

[0014] For example, because it is easy to correct the angle of imagerotation and X-ray observation is possible, the scanning electronmicroscope can be used in such a manner that it selects the secondlow-magnification mode when normal low-magnification observation isperformed, and it automatically switches to the first low-magnificationmode if observation is to be performed under lower magnification.

[0015] In addition, it is desirable to have the scanning electronmicroscope configured in such a manner that it has storage means thateach stores setting values of brightness and contrast of the sampleimage independently for the high-magnification mode, the firstlow-magnification mode, and the second low-magnification mode, andaccording to switching to each of the magnification modes, the settingvalues of brightness and contrast for each of the magnification modesare automatically set to be the values stored in the storage means.

[0016] It is desirable that the deflecting means have a function ofcontrolling the scanning direction of the primary electron beam, and thescanning direction of the primary electron beam be controlled accordingto switching between high-magnification mode, the firstlow-magnification mode, and the second low-magnification mode. It isdesirable that the control of the scanning direction of the primaryelectron beam by the deflecting means be performed in such a way thatthe scanning direction of the primary electron beam on the samplesubstantially corresponds to the X direction of the sample stage.

[0017] When sample observation is to be performed by using the scanningelectron microscope disclosed in the embodiments of the presentinvention, the setting value of the exciting current of the object lensin the first low-magnification mode is usually set to be a value forweak excitation, and the value is switched to zero when the scannedimage of the sample is to be recorded. The scanned image of the sampleis recorded by taking an image shown on a display, storing or outputtingthe scanned image of the sample as a file.

[0018] In addition, when sample observation is to be performed,observation of an X-ray mapping image in low-magnification mode isperformed in the second low-magnification mode.

[0019] A scanning electron microscope disclosed in embodiments of thepresent invention comprises an electron source, a first focusing lensfor focusing a primary electron beam emitted from the electron source,an object lens diaphragm for removing an unnecessary region of theprimary electron beam focused by the first focusing lens, a secondfocusing lens for focusing the primary electron beam that has passedthrough the object lens diaphragm, an object lens for generating amagnetic field at the position of a sample and for focusing the primaryelectron beam focused by the second focusing lens on the sample, anelectron beam deflecting means for the scanning of the sample by theprimary electron beam, and an X-ray detector for detecting an X-rayemitted from the sample due to electron beam irradiation, whereby anX-ray mapping image of the sample is obtained. The primary electron beamis focused on the sample by the object lens to perform scanning when themagnification of an image to be scanned is higher than a preset value,and the primary electron beam is focused on the sample by the secondfocusing lens to perform scanning when the magnification of an image tobe scanned is lower than a preset value. The scanning electronmicroscope has a configuration that sets the exciting current of theobject lens to be in a weak excitation state to prevent the incidence ofa reflected electron from the sample on the X-ray detector.

[0020] Here, if the magnification of an image to be scanned is lowerthan the preset value, the exciting current of the object lens ischanged in proportion to the square root of the accelerating voltage ofthe primary electron beam.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a schematic view of an example of a scanning electronmicroscope according to the present invention.

[0022]FIG. 2 shows a relation between an object lens exciting currentand secondary electron detection efficiency/brightness region inlow-magnification mode.

[0023]FIGS. 3A and 3B are diagrams of assistance in explaining thepresence and the absence of an object lens magnetic field and thetrajectories of reflected electrons.

[0024] FIG.4 is a diagram of assistance in explaining the X-ray spectrumdetected by an X-ray detector when a reflected electron falls on theX-ray detection plane of the X-ray detector.

[0025]FIG. 5 is a flowchart showing an example of a process for X-rayanalysis.

[0026]FIG. 6 is a diagram of assistance in explaining image rotationcaused by changes in the excitation of an object lens.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] Preferred embodiments of the present invention will now bedescribed.

[0028]FIG. 1 is a schematic view of an example of a scanning electronmicroscope according to the present invention. An extracting voltage isapplied between a cathode 2 and a first anode 3 located in an electronsource 1 by a high-voltage control power supply 11, so that a primaryelectron beam 5 is emitted from the cathode 2. The primary electron beam5 is accelerated by the voltage applied between the cathode 2 and asecond anode 4 by the high-voltage control power supply 11 and proceedsto a lens system in the next stage. The primary electron beam 5 is thenfocused by a first focusing lens 6 (C1 lens) fed by a lens control powersupply 13, and passes through an object lens diaphragm 7, whereby anunnecessary region of the beam is removed.

[0029] The primary electron beam 5 that has passed through the objectlens diaphragm 7 is focused by a second focusing lens 8 (C2 lens) fed bya lens control power supply 15, and is then focused on a sample 26 by anobject lens 10 driven by an object lens driving power supply 18.Deflecting coils 9 a and 9 b, which are driven by a scanning powersupply 16, are arranged in two stages between the C2 lens 8 and theobject lens 10. The deflecting coils 9 a and 9 b allow the primaryelectron beam 5 to scan the sample 26 in a two-dimensional manner.

[0030] A beam alignment coil 22, which is driven by a beam alignmentcoil control power supply 12, is a coil for magnetically correcting thetrajectory of the primary electron beam (adjusting the optical axis ofthe primary electron beam), while an astigmatic coil 23, which is drivenby an astigmatic coil control power supply 14, is a coil formagnetically correcting image distortion due to astigmatism.

[0031] A secondary electron 24 generated from the sample 26 passesthrough the object lens 10. Then the secondary electron 24 is deflectedto the secondary electron detector side by an orthogonal electromagneticfield generator 27 placed above the object lens, and is detected by asecondary electron detector 25. In the orthogonal electromagnetic fieldgenerator 27, there is created a field where the optical axis, theelectric field, and the magnetic field are made orthogonal to eachother, and the intensities of the electric field and the magnetic fieldare set in such a manner that the electromagnetic field does not producethe deflection effect on the primary electron beam, whereas theelectromagnetic field deflects the secondary electron 24, which proceedsin the direction opposite to the direction of the primary electron beam5, toward the secondary electron detector 25.

[0032] The detection signal of the secondary electron detector 25 isamplified by a signal amplifier 41, is processed by a signal processingmeans 20, and is displayed on a CRT 21 as a sample image, while ifnecessary, the detection signal of the secondary electron detector 25 isstored in an image memory 28 as an image signal.

[0033] In normal high-resolution observation (high-magnification mode),the primary electron beam 5 is focused on the sample by the object lens10 in the manner as described above. However, in high-resolutionobservation conditions involving a short distance between the objectlens 10 and the sample 26, in particular, the distance from thedeflection point to the sample 26 is also shortened, and therefore themaximum observation view (scanning region) to be obtained inhigh-magnification mode cannot be widened. Thus, to further widen theobservation view, the scanning region on the sample 26 is enlarged byswitching to the scanning by means of a one-stage deflecting coil.

[0034] In this case, if the object lens 10 is strongly excited, as inhigh-magnification mode, the displayed image is greatly distorted due tolens aberration. Therefore, the primary electron beam 5 is focused onthe sample by putting the object lens 10 into a zero excitation state ora weakly excited state and by using the C2 lens 8 (firstlow-magnification mode).

[0035] In X-ray analysis, X-rays 32 generated from the sample 26 fall onthe detection plane of an X-ray detector 33 placed in the proximity ofthe sample, and the detection signal of the X-ray detector 33 isamplified by a signal amplifier 42, is processed by the signalprocessing means 20, and is displayed on the CRT 21 as an X-ray spectrumand an X-ray mapping image.

[0036] In normal high-resolution observation, lens control is performedin high-magnification mode, as in the case of the observation of asecondary electron image. If X-ray observation is to be performed with aview wider than the maximum observation view obtained inhigh-magnification mode, the exciting current of the object lens 10 isset to be a value in proportion to the square root of the acceleratingvoltage, and the primary electron beam 5 is focused on the sample byusing the C2 lens 8 (second low-magnification mode).

[0037] Each of the power supplys 11 to 18 mentioned above is controlledby a CPU 19. The CPU 19 also controls the signal processing means 20. Inaddition, the CPU 19 is connected with a memory (storage area) 29 to beused exclusively for high-magnification mode and a memory (storage area)30 to be used exclusively for low-magnification mode. The memory 29exclusively for high-magnification mode stores the values of the signalamplifier 41 and the signal processing means 20 that are set withrespect to the brightness and the contrast of a sample image when itsobservation is performed in high-magnification mode. The memory 30exclusively for low-magnification mode stores the values of the signalamplifier 41 and the signal processing means 20 that are set withrespect to the brightness and the contrast of a sample image when itsobservation is performed in the first low-magnification mode, as well asthe values of the signal amplifier 41 and the signal processing means 20that are set with respect to the brightness and the contrast of a sampleimage when its observation is performed in the second low-magnificationmode.

[0038] Next, the first low-magnification mode and the secondlow-magnification mode will be described. FIG. 2 shows a relationbetween the object lens exciting current and the secondary electrondetection efficiency/brightness region in low-magnification mode. FIGS.3A and 3B show diagrams of assistance in explaining the presence and theabsence of an object lens magnetic field and the trajectories ofreflected electrons. FIG. 4 shows a diagram of assistance in explainingthe X-ray spectrum detected by the X-ray detector when a reflectedelectron falls on the X-ray detection plane of the X-ray detector.

[0039] In general, if the exciting current of the object lens is variedin lens control, the amount of detection signal of a secondary electronand the brightness region of the image is changed in the manners asshown in FIG. 2. The amount of detection signal of a secondary electronrepresents the brightness of the sample image. As the object lenscurrent is increased, the amount of detection signal increases due tothe winding-up effect of the magnetic field of the object lens,resulting in a bright image. However, the amount of detection signalreaches convergence at a certain current value. On the other hand, thebrightness region of the image determines the lowest magnification ofthe image. When the exciting current of the object lens is zero, thewidest view and a sample image with no variations in brightness can beobtained. However, as the object lens current is increased, thebrightness region of the image is reduced, with the result that theperiphery of the image becomes dark.

[0040] Therefore, the exciting current value of the object lens in thefirst low-magnification mode is set to be a value that can maximize thebrightness region of the image, chosen from among the current values atwhich the amount of detection signal of a secondary electron isconverged. If the microscope is used specifically for taking sampleimages or the like, the exciting current value of the object lens is setto be zero so that the brightness region of an image is maximized. Theexciting current value of the object lens is set to be zero for thefollowing reason. When the exciting current value of the object lens iszero, the amount of detection signal of a secondary electron is small,but if the microscope is used only for taking sample images, the smallamount of detection signal of a secondary electron can be compensatedfor by setting the exposure time or the signal accumulation time to belong.

[0041] On the other hand, the second low-magnification mode is thelow-magnification mode for X-ray analysis, and the current value in thesecond low-magnification mode is changed within the range of currentvalues that are higher than the exciting current value of the objectlens set in the first low-magnification mode. In X-ray analysis, anobject lens magnetic field needs to be-generated according to the energyof a reflected electron (=accelerating voltage) to make the trajectoryof the reflected electron go away from the detection plane of the X-raydetector. Therefore the current value is set in proportion to the squareroot of the accelerating voltage.

[0042] In normal low-magnification mode (the first low-magnificationmode), the magnetic field of the object lens 10 is not present or weak,as shown in FIG. 3B, and therefore the reflected electron 35 generatedfrom the sample 26 falls on the detection plane of the X-ray detector33. The X-ray spectrum detected by the X-ray detector 33 in this case issuch as is shown in FIG. 4. It is difficult to perform X-ray analysis inthis case because the reflected electron 35 incident on the detectionplane of the X-ray detector 33 causes a detection error or a failure.Therefore, in the low-magnification mode for X-ray analysis, theexciting current value of the object lens is set in such a way that thereflected electron 35 goes away from the detection plane of the X-raydetector 33 and the brightness region of the image is secured by settingthe above value to be the lowest current value, as shown in FIG. 3A. Thehigher the accelerating voltage of the primary electron beam is, thegreater the exciting current of the object lens needs to be, andtherefore the brightness region of the image is reduced in responsethereto.

[0043] Moreover, in normal observation of a secondary electron image bymeans of the scanning electron microscope, the accelerating voltageduring the observation of the same sample is constant in many cases,while during X-ray observation, the accelerating voltage needs to bechanged to perform X-ray observation. The accelerating voltage needs tobe changed for the following reason. Of the elements that form thesample, light elements emit X-rays by irradiating with a primaryelectron beam accelerated to a low velocity, while heavy elementsrequire irradiation with a primary electron beam accelerated to a highvelocity. Thus, even when the same sample is to be observed, theaccelerating voltage of the primary electron beam needs to be changedduring its observation, depending on the elements to which attention isdirected.

[0044] The primary electron beam proceeds in a spiral trajectory in themagnetic field of the object lens to be focused on the sample. If theaccelerating voltage is changed or the exciting current is changed here,the trajectory of the primary electron beam is changed, therebyresulting in a rotation of the image. However, if excitation IN/{squareroot}{square root over (V)}(I: exciting current, N: number of coilturns, V: accelerating voltage) is held constant, the trajectory of theprimary electron beam is not changed, and therefore the rotation of theimage can be readily controlled. Since the number of coil turns N is aconstant value specific to the object lens, the excitation can be heldconstant at all times by changing the exciting current I in proportionto the square root of the accelerating voltage V. Similarly, themagnification of the image can be readily controlled by changing thecurrent of the deflecting coil for scanning the primary electron beam inproportion to the square root of the accelerating voltage V.

[0045] Here, in brief, supplementary description of the excitation ofthe object lens will be made. In high-magnification mode, the primaryelectron beam 5 is controlled by the magnetic field generated by theobject lens 10 in such a way that the primary electron beam 5 is focusedon the sample 26. In this case, a strong exciting current flows throughthe object lens 10, enabling short focusing. Thereforehigh-magnification observation can be performed. Such excitation of theobject lens 10 is called strong excitation.

[0046] In the first low-magnification mode, the primary electron beam 5is controlled in such a way that the exciting current of the object lens10 is set to be zero or weak and the primary electron beam 5 is focusedon the sample 26 by the focusing lens (second focusing lens 8) nearestto the object lens 10. This makes it possible to enlarge the brightnessregion and therefore perform an observation with a wide view(low-magnification observation). In this case, the exciting currentflowing through the object lens 10 is lower than that inhigh-magnification mode, and accordingly it cannot cause the primaryelectron beam 5 to be focused on the sample. In the secondlow-magnification mode, the exciting current of the object lens 10 iscontrolled in such a way that the exciting current changes in proportionto the square root of the accelerating voltage V. The exciting currentin the second low-magnification mode is greater than that in the firstlow-magnification mode but lower than that in high-magnification mode.As in the first low-magnification mode, the object lens 10 cannot causethe primary electron beam 5 to be focused on the sample 26 (weakexcitation). In the second low-magnification mode, the incidence of thereflected electron 35 on the X-ray detection plane of the X-ray detector33 can be restricted by the magnetic field of the object lens generatedby an exciting current greater than that of the first low-magnificationmode.

[0047]FIG. 5 is a flowchart showing an example of the process for X-rayanalysis. When sample observation is to be performed, the processing isready for the first low-magnification mode at a step 11 to perform thesetting of C2 lens (second focusing lens) conditions, axis adjustment,focus adjustment, and brightness and contrast adjustment. The brightnessadjustment is performed by setting the value of the signal amplifier 41for the secondary electron detector 25 to be the value for the firstlow-magnification mode stored in advance in the memory 30. The contrastadjustment is performed by setting the value of the signal processingmeans 20 to be the value for the first low-magnification mode stored inadvance in the memory 30. Next, at a step 12, a view search is performedunder low magnification.

[0048] After the observation view is determined, switching tohigh-magnification mode is performed at a step 13. In high-magnificationmode, the deflectors 9 a and 9 b are used in two stages, and the objectlens 10 is used in a strong excitation state. After proceeding tohigh-magnification mode, the setting of C2 lens conditions, axisadjustment, focus adjustment, and brightness and contrast adjustment areperformed at a step 14. The brightness adjustment is performed bysetting the value of the signal amplifier 41 for the secondary electrondetector 25 to be the value for high-magnification mode stored inadvance in the memory 29. The contrast adjustment is performed bysetting the value of the signal processing means 20 to be the value forhigh-magnification mode stored in advance in the memory 29.

[0049] After switching to high-magnification mode, the image is observedunder high magnification by means of the scanning electron microscope ata step 15. In addition, X-ray analysis under high magnification isperformed at a step 16. Specifically, the X-ray spectrum of a localregion of the sample and a local X-ray mapping image are observed.

[0050] Next, at a step 17, switching to the second low-magnificationmode is performed. In the second low-magnification mode, one of thedeflectors is used in one stage, and the object lens 10 is used in aweak excitation state in which the exciting current of the object lens10 is caused to change in proportion to the square root of theaccelerating voltage of the primary electron beam 5. After proceeding tothe second low-magnification mode, the setting of C2 lens conditions,axis adjustment, focus adjustment, and brightness and contrastadjustment are performed at a step 18. The brightness adjustment isperformed by setting the value of the signal amplifier 41 for thesecondary electron detector 25 to be the value for the secondlow-magnification mode stored in advance in the memory 30. The contrastadjustment is performed by setting the value of the signal processingmeans 20 to be the value for the second low-magnification mode stored inadvance in the memory 30.

[0051] Next, at a step 19, X-ray analysis under low magnification, thatis, observation of the whole X-ray mapping image of the sample isperformed. In this second low-magnification mode, the reflected electrongenerated from the sample is caused to go away from the X-ray detector33 by the magnetic field of the object lens 10 in a weakly excitedstate, and therefore does not fall on the X-ray detector 33. Thus, anelement mapping image can be observed under low magnification without afailure that may occur in the X-ray detector 33 or without noiseincluded in the detection signal as in the case of a conventionalmicroscope.

[0052] Now, if the excitation of the object lens 10 is changed byswitching between magnification modes, the rotation effect of the objectlens is produced on the scanning direction of the primary electron beam,thereby resulting in a rotated image. FIG. 6 shows a diagram ofassistance in explaining the relation between the object lens excitingcurrent and the image rotation. According to the present invention, thescanning direction of the primary electron beam is controlled in such away that the image rotation caused by the object lens is cancelled inassociation with the excitation of the object lens, to make the scanningdirection of the primary electron beam correspond to the X direction ofthe sample stage.

[0053] In the second low-magnification mode, the microscope is usedunder the condition of constant excitation IN/{square root}{square rootover (V)}(I: exciting current, N: number of coil turns, V: acceleratingvoltage). Therefore the trajectory of the primary electron beam is notchanged in the magnetic field of the object lens, and so the rotation ofthe image does not occur. Also in high-magnification mode, the rotationof the image will not occur if the microscope is used under thecondition of constant excitation IN/{square root}{square root over (V)}.However, if the microscope is used under a condition that the excitationis not held constant, the rotation of the image will occur when thecurrent value of the object lens or the like is changed. In the firstlow-magnification mode, the microscope is used under the condition thatthe exciting current of the object lens is held constant, and thereforethe rotation of the image will occur when the accelerating voltage ofthe primary electron beam is changed. Thus, in observation modes wherethe excitation IN/{square root}{square root over (V)}is not heldconstant, the scanning direction of the primary electron beam iscontrolled in such a way that the image rotation caused by the objectlens can be cancelled, by adjusting the ratio of the deflecting currentflowing through the X deflecting coils of the deflecting coils 9 a and 9b to the deflecting current flowing through the Y deflecting coils ofthe deflecting coils 9 a and 9 b. As a result, the scanning direction ofthe primary electron beam on the sample is made to coincide with the Xdirection of the sample stage at all times.

[0054] According to the embodiments of the present invention, efficientlow-magnification observation is made possible by setting a plurality oflow-magnification modes each sutable for sample image (secondaryelectron image) observation and for X-ray analysis. The presentinvention makes it possible to use low-magnification mode during X-rayanalysis, in particular, which has previously been difficult, and toobtain an X-ray mapping image with a wide view.

What is claimed is:
 1. A scanning electron microscope comprising: anelectron source; a first focusing lens for focusing a primary electronbeam emitted from said electron source; an object lens diaphragm forremoving an unnecessary region of the primary electron beam focused bysaid first focusing lens; a second focusing lens for focusing theprimary electron beam that has passed through said object lensdiaphragm; an object lens for focusing the primary electron beam focusedby said second focusing lens on a sample; a deflecting means for thescanning of the sample by the primary electron beam; a secondaryelectron detector for detecting a secondary electron emitted from thesample due to electron beam irradiation; and an X-ray detector fordetecting an X-ray emitted from the sample; wherein said scanningelectron microscope has a function of focusing the primary electron beamon the sample by using said object lens when the magnification of animage to be scanned is higher than a preset value (high-magnificationmode), wherein said scanning electron microscope has a function offocusing the primary electron beam on the sample by using said secondfocusing lens when the magnification of an image to be scanned is lowerthan a preset value (low-magnification mode), and wherein, in saidlow-magnification mode, either a first low-magnification mode in whichthe exciting current of said object lens is set to be a constant valueindependently of the accelerating voltage of the primary electron beamor a second low-magnification mode in which the exciting current of saidobject lens is changed as a function of the accelerating voltage of theprimary electron beam is selected.
 2. A scanning electron microscope asclaimed in claim 1, wherein said sample is placed in the magnetic fieldof said object lens.
 3. A scanning electron microscope as claimed inclaim 1, wherein said first low-magnification mode sets the excitingcurrent of said object lens to be zero or in a weak excitation state. 4.A scanning electron microscope as claimed in claim 1, wherein said firstlow-magnification mode sets the exciting current of said object lens tobe either the minimum exciting current of said object lens that does notlower the efficiency of secondary electron detection or the excitingcurrent of said object lens that provides the maximum view for theobservation magnification.
 5. A scanning electron microscope as claimedin claim 1, wherein the exciting current of said object lens in saidsecond low-magnification mode is set to be in proportion to the squareroot of the accelerating voltage of the primary electron beam.
 6. Ascanning electron microscope as claimed in claim 1, wherein switchingbetween said first low-magnification mode and said secondlow-magnification mode is performed automatically according to the setvalue of observation magnification.
 7. A scanning electron microscope asclaimed in claim 1, wherein said scanning electron microscope hasstorage means that each store setting values of brightness and contrastof said sample image independently for said high-magnification mode,said first low-magnification mode, and said second low-magnificationmode, and according to switching to each of said magnification modes,the setting values of brightness and contrast for each of saidmagnification modes are automatically set to be the values stored insaid storage means.
 8. A scanning electron microscope as claimed inclaim 1, wherein said deflecting means controls the scanning directionof said primary electron beam according to switching between saidhigh-magnification mode, said first low-magnification mode, and saidsecond low-magnification mode.
 9. A scanning electron microscope asclaimed in claim 8, wherein said deflecting means controls the scanningdirection of said primary electron beam in such a way that the scanningdirection of said primary electron beam on said sample substantiallycorresponds to the X direction of a sample stage on which said sample ismounted.
 10. A sample observation method by means of a scanning electronmicroscope, comprising the steps of: focusing a primary electron beam ona sample by using a focusing lens when the magnification of an image tobe scanned is lower than a preset value; setting the exciting current ofan object lens to be a constant value for weak excitation independentlyof the accelerating voltage of said primary electron beam; and switchingthe exciting current of the object lens to zero when the scanned imageof the sample is to be recorded.
 11. A sample observation method bymeans of a scanning electron microscope, comprising the steps of:focusing a primary electron beam on a sample by using a focusing lenswhen the magnification of an image to be scanned is lower than a presetvalue; and changing the exciting current of an object lens as a functionof the accelerating voltage of said primary electron beam to observe theX-ray mapping image of said sample.
 12. A scanning electron microscopecomprising: an electron source; a first focusing lens for focusing aprimary electron beam emitted from said electron source; an object lensdiaphragm for removing an unnecessary region of the primary electronbeam focused by said first focusing lens; a second focusing lens forfocusing the primary electron beam that has passed through said objectlens diaphragm; an object lens for generating a magnetic field at theposition of a sample and for focusing the primary electron beam focusedby said second focusing lens on the sample; an electron beam deflectingmeans for the scanning of the sample by the primary electron beam; andan X-ray detector for detecting an X-ray emitted from the sample due toelectron beam irradiation; whereby an X-ray mapping image of the sampleis obtained, wherein the primary electron beam is focused on the sampleby said object lens to perform scanning when the magnification of animage to be scanned is higher than a preset value, and the primaryelectron beam is focused on the sample by said second focusing lens toperform scanning when the magnification of an image to be scanned islower than a preset value, and wherein the exciting current of saidobject lens is set to be in a weak excitation state to prevent theincidence of a reflected electron from the sample on said X-raydetector.
 13. A scanning electron microscope as claimed in claim 12,wherein if the magnification of said image to be scanned is lower thanthe preset value, the exciting current of said object lens is changed inproportion to the square root of the accelerating voltage of the primaryelectron beam.